Fits most bottles and fits via the vent or blank, just a few quid.
Flomasta Universal Filling Kit 5 Pcs - Screwfix
Order online at Screwfix.com. Reusable kit that helps feed treatment chemicals directly into central heating systems. For 500ml and 1Ltr bottles and 310ml concentrate tubes from all major brands including Fernox, Sentinel, Flomasta and No Nonsense. Bottles can be filled through ½" BSP radiator...www.screwfix.com
Looks good but can't see how it would pump the stuff in overcoming the pressure from the F/E tank ?